These and other works searched for differences between the gas content of AGN and non-active galaxies in the hope of finding evidence for quenching induced by AGN feedback. However, most of these comparisons show that the AGN gas fractions are equal or larger than those of non-active galaxies matched in stellar mass, indicating that when the AGN is on, generally there is gas and star formation (although this depends on galaxy morphology, see e.g. the QSO2s in red ETGs). This does not mean that AGN feedback is not having an effect on the molecular gas reservoirs of galaxies, but considering the short AGN lifetimes, and the possibility that every massive galaxy goes through one or several active phases, spotting the action of feedback on the total gas fractions becomes challenging.
The region we should focus on if we aim to evaluate the effect of the current episode of nuclear activity is the central kpc. This is the region having a dynamical timescale comparable to the periods of nuclear activity and the outflows (\(\sim \) 1–100 Myr; see e.g. [1]}). Furthermore, reduced gas fractions relative to star-forming regions in the same host galaxies have been recently measured in the central kpc of AGN using CO observations [2]}. In our QSO2s, we find smaller concentrations of molecular gas in the central kpc of the spiral galaxies, which are the ones showing more massive molecular outflows and higher Eddington ratios (see Section REF ). In the merging QSO2s, we see tentative evidence of AGN feedback modifying the distribution of CO in the central 1-2 kpc of the galaxies (i.e. double-peaked CO morphologies in J1010 and J1430). This could be consistent with an inside-out quenching scenario (e.g. [3]}).
